As computers have become increasingly powerful in terms of their data processing capabilities, the desire for systems that are capable of storing and retrieving large amounts of data in a reliable and rapid manner has correspondingly increased. Although volatile memory devices that are capable of storing data only when power is provided to those devices are of importance in meeting this need, non-volatile memory devices that do not require continued energy input in order to maintain the recorded data also continue to be of importance.
Most non-volatile memory devices used today involve one of two general technologies, namely, magnetic recording media and optical recording media. Although both technologies have improved over the years in terms of their reliability, speed, and capacity for data storage, there continues to be a need for non-volatile memory devices with even greater performance. In particular with respect to improving the amount of data that can be stored on a given device, there remains a need for improvements in the amount of digital information that can be stored per unit area (“areal storage density”) of the substrates of these recording media.
Specifically with respect to magnetic recording devices, limitations in the long-term stability of very small magnetic domains, and the manufacturing and operational tolerances of the read-write heads employed to interface the magnetic substrates on which data is recorded, presently preclude areal storage densities of greater than about 70–90 Gb/in2 in magnetic recording media. Although further developments in “perpendicular recording” technologies have the potential for improving the areal storage densities of magnetic recording media beyond 100 Gb/in2, this has not yet been demonstrated, as discussed in “Hard Disk Recording Aims To Get Perpendicular” by Peter Singer in the September 2002 issue of Semiconductor International (pg 50-54), which is hereby incorporated by reference herein.
As for optical recording devices, these devices in recent years have lagged behind magnetic recording devices in terms of their achievable areal storage densities. The present near-term target for the areal storage densities of optical digital versatile disks (DVDs) is only about 40–50 Gb/in2.
Although optical recording devices have fallen behind magnetic recording devices in terms of their achievable areal storage densities, optical recording devices have certain advantages in comparison with magnetic storage devices that continue to make optical recording devices attractive. In particular, optical recording media are able to reliably store data for much longer time periods (e.g., decades) than magnetic recording media. Also, the light sources and detectors used to store and retrieve data recorded on optical recording media can be positioned significantly farther away from the substrates on which data is recorded than the read-write heads used to store and retrieve data recorded on magnetic recording media. Consequently, optical recording devices are easier to design and somewhat more robust than magnetic recording devices.
The areal storage density of an optical recording medium is largely determined by how small one can make the size of the area or spot that is illuminated on the substrate to store and retrieve information without excessive bit error rates. This “illumination spot size” in turn is determined by the illumination wavelength (diffraction limit) and the numerical aperture of the imaging optics, as discussed generally in “Everything You Wanted To Know About DVD-R.(Technology Information)” by Andy Parsons in the February 2000 issue of Computer Technology Review and “Optical Data Storage” by H. Coufal and G. W. Burr in the 2002 issue of International Trends in Applied Optics (Chapter 26), each of which is hereby incorporated by reference herein. More specifically, the illumination spot size (A) and maximum areal storage density (D*) in bits per unit area (b) of an optical storage medium respectively depend upon wavelength (λ) and numerical aperture (NA) as follows:A˜(λ/NA)2  (1)D*˜b/A  (2)
In view of the relationships expressed in Equations (1) and (2), one approach for increasing areal storage density D* is to reduce the illumination wavelength and thereby reduce the illumination spot size. This is quite effective, since both the illumination spot size and the areal storage density depend quadratically upon on the wavelength employed.
One cutting-edge attempt at attaining an improved areal storage density in this manner involves substituting a conventional, 820 nm wavelength (red) light source such as a GaAs laser with a 460 nm wavelength (blue) light source such as a blue diode AlGaN laser, as discussed in the article “Blue Laser CD Technology” in Scientific American, July, 1996. pp. 48–51, which is hereby incorporated by reference herein. Given that the successful implementation of an optical recording device utilizing a 460 nm wavelength light source would offer nearly a 50% reduction in wavelength vis-à-vis the conventional 820 nm wavelength light source, such an implementation would result in a maximum achievable areal storage density of nearly four times that of conventional devices. Although optical recording devices utilizing blue lasers continue to be pursued, such devices are not yet commercially available.
Theoretically, the areal storage densities of optical recording media can continue to increase as the wavelengths of the light employed in conjunction with those media continue to be decreased below the wavelengths associated with blue light. Practically, however, the use of light of such shorter wavelengths becomes problematic for at least two reasons. First, successful implementation of an optical recording device requires a light source that provides light that is reasonably monochromatic at the wavelength of interest, and that is of sufficient intensity to allow the attainment of satisfactory signal-to-noise ratios. To the extent that it is desired to be able to write data onto the optical storage media, and not just read data, a light source of even stronger intensity is required. At the wavelengths of light at which conventional optical recording devices typically operate, lasers are the only light sources that can effectively provide these characteristics. Yet, economical, solid state lasers providing light at wavelengths lower than those associated with blue light continue to be unavailable.
Second, it is well known that the atmosphere is strongly absorbing for radiation at wavelengths of less than about 190 nm in the “vacuum ultraviolet” range of the electromagnetic spectrum (which is generally defined as including light with wavelengths between about 10 nm and about 190 nm), such that nearly total absorption of such light occurs with transmission through only several millimeters of air at atmospheric pressure. The term “light” when used herein generally is not intended to be limited to a specific range of the electromagnetic spectrum such as the “visible light” region, but rather is intended to encompass radiation of a variety of wavelengths and, in particular, is intended to encompass radiation including the vacuum ultraviolet range of the electromagnetic spectrum. Consequently, light at wavelengths of less than 190 nm is seldom used in practice in optical devices, except in optical devices in which it is practical to limit the transmission of the light through sealed passage(s) that have been evacuated and/or inert gas purged. Because it is excessively expensive and otherwise impractical to utilize such sealed passages in optical recording devices, the use of light at wavelengths in the vacuum ultraviolet range appears to be generally impractical for optical recording devices.
Despite these difficulties associated with optical recording devices using light at shorter wavelengths, it nevertheless would be advantageous if a new optical recording device was developed that could achieve significantly higher areal storage densities than conventional optical recording devices or even cutting-edge optical recording devices that may employ blue light. It would further be advantageous if such a new optical recording device attained areal storage densities that rivaled or even exceeded those of conventional magnetic recording devices, and at the same time still provided the advantages associated with optical recording devices in comparison with magnetic recording devices. Additionally, it would be advantageous if such a new optical recording device did not require the use of vacuum-sealed passages for transmitting light to and from the optical storage media, could operate using a conventional light source, and otherwise was not significantly more complicated or expensive to design or manufacture than conventional optical recording devices.